Temperature probe and method of making the same

ABSTRACT

A temperature probe assembly is provided. The temperature probe assembly may comprise a housing formed of a first thermally conductive material and having an inner diameter defined by an inner bore, an insert formed of a second thermally conductive material disposed in the inner bore and having an outer diameter that is substantially equal to the inner diameter of the housing at a first temperature and a temperature sensor mounted within the insert. The second thermally conductive material has a thermal coefficient of expansion that is greater than the first thermally conductive material, such that the insert is insertable into the inner bore at the first temperature and is tightly locked in the inner bore at a second temperature that is greater than the first temperature.

This invention was made with Government support under Contract No.F29601-97-C-0001 awarded by Aeronautical Systems Command. The Governmenthas certain rights in this invention.

TECHNICAL FIELD

The present invention relates generally to temperature probes, and moreparticularly to a temperature probe employing a resistive temperaturedevice (RTD) element mounted within a protective housing.

BACKGROUND

Temperature probes are used in many applications for sensing thetemperature of a solid, liquid or gas. For example, temperature probesare used in certain high energy laser systems to detect the temperatureof basic hydrogen peroxide (BHP), iodine, chlorine and other chemicalsused in the generation of the laser beam. Other applications includemedical, pharmaceutical, food, chemical, aerospace and industrialapplications. In certain applications, such as high energy lasersystems, it is important to measure certain temperatures very accuratelyand very quickly.

Different classes of temperature sensors are known in the art to measuretemperature. One class of temperature sensors employs resistiveelements, well known to those skilled in the art. As the temperature ofthe element increases or decreases, the resistance of the element alsoincreases or decreases providing an indication of the temperaturechange. A constant precision current signal is applied to the resistanceelement resulting in a voltage drop across the element proportional toits resistance and the temperature it is subjected to. The voltage isthen measured to give a reading of the resistance, and thus thetemperature corresponding with that particular resistance.

Known temperature probes that employ resistive elements typically have aresponse time (time constant) of several seconds. Particularly, when thetemperature of the environment that the sensor is sensing changes, thesensor does not give the exact temperature reading for the change untilmore than several seconds later. The probe response time is definedherein as the time it takes the temperature sensor to respond through63.2% of the total temperature change. This slow of a response time isunacceptable in many applications. The slow response time can beattributed to the fact that the resistive element is mounted within aprotective housing that typically includes pockets of air and bondingagents between the element and the housing. The pockets of air canconsiderably reduce the thermal conductivity between the media and theresistive element resulting in a slower time constant of the probe.

SUMMARY

In one aspect of the invention, a temperature probe assembly isprovided. The temperature probe assembly comprises a housing formed of afirst thermally conductive material and having an inner diameter definedby an inner bore, an insert formed of a second thermally conductivematerial disposed in the inner bore and having an outer diameter that issubstantially equal to the inner diameter of the housing at a firsttemperature and a temperature sensor mounted within the insert. Thesecond thermally conductive material has a thermal coefficient ofexpansion that is greater than the first thermally conductive material,such that the insert is insertable into the inner bore at the firsttemperature and is tightly locked in the inner bore at a secondtemperature that is greater than the first temperature.

In another aspect of the invention, a method for fabricating atemperature probe assembly is provided. The method comprises forming aninner bore in a housing of a first thermally conductive material, theinner bore having an internal diameter at a first temperature, formingan insert of a second thermally conductive material having an outerdiameter that is substantially equal to the internal diameter at thefirst temperature and an inner cavity dimensioned to house a temperaturesensor, and bonding a temperature sensor in the inner cavity. The methodfurther comprises inserting the insert into the inner bore of thehousing at the first temperature to form a temperature probe assembly,and exposing the temperature probe assembly to a second temperature thatis greater than the first temperature. The second thermally conductivematerial has a thermal coefficient of expansion that is greater than thefirst thermally conductive material, such that the insert is tightlylocked in the inner bore at the second temperature.

In yet another aspect of the present invention, a method for fabricatinga temperature probe assembly is provided. The method comprises machiningan inner bore in a housing of a first thermally conductive material, theinner bore having an internal diameter at a first temperature having arange of about −40° F. (−40° C.) to about −100° F. (−73.33° C.) andmachining or stamping an insert of a second thermally conductivematerial having an outer diameter that is substantially equal to theinternal diameter at the first temperature and an inner cavitydimensioned to house a resistive temperature device (RTD) element. Themethod further comprises bonding the RTD element in the inner cavity,coupling signal wires to the RTD element and inserting the insert intothe inner bore of the housing at the first temperature to form atemperature probe assembly. The temperature probe assembly is thenexposed to a second temperature greater than or equal to about 32° F.(0° C.), wherein the second thermally conductive material has a thermalcoefficient of expansion that is greater than the first thermallyconductive material, such that the insert is tightly locked in the innerbore at the second temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of a temperature probeassembly in accordance with an aspect of the present invention.

FIG. 2 illustrates a side view of a cylindrical insert in accordancewith an aspect of the present invention.

FIG. 3 illustrates a plan view of the cylindrical insert of FIG. 2.

FIG. 4 illustrates a methodology for fabricating a temperature probeassembly in accordance with an aspect of the present invention.

DETAILED DESCRIPTION

The present invention relates to a temperature sensor probe and a methodof making the same. The temperature sensor probe can be operative formeasuring the temperatures of corrosive media, such as molten iodine,chlorine, ammonia, basic hydrogen peroxide and others very accuratelyand extremely fast without being degraded or destroyed by the media. Thetemperature sensor probe employs a thermally conductive insert thathouses a resistive temperature device (RTD) element (e.g., a precisionthin film sensor device). The thermally conductive insert can beinserted into a central longitudinal bore of a thermally conductivehousing (or sheath) through an open end to a closed end of the housingto mate with an internal diameter of the housing.

Both the internal diameter of the housing and an external diameter ofthe insert can be machined to be of a substantially same orsubstantially equal diameter at a first temperature (e.g., about −40° F.(−40° C.) to about −100° F. (−73.33° C.)). The thermally conductivematerial of the insert is selected to have a thermal coefficient ofexpansion that is greater than the thermally conductive material of thehousing. Therefore, the insert can be inserted into the housing at thefirst temperature to form a temperature probe assembly. The temperatureprobe assembly can then be removed from the first temperature andexposed to a second temperature (e.g., a room temperature that isgreater than or equal to about 32° F. (0° C.)) that is higher than thefirst temperature.

As the ambient temperature rises, the insert will expand at a fasterrate than the housing resulting in an extremely tight fit at elevatedtemperatures and a repeatable fast temperature constant. The presentinvention provides for high yields (e.g., 95%), fast performance (e.g.,at least about 250 msec response time or faster for single probes), andsubstantially lower costs than current implementations. The secondtemperature can be selected to allow the insert to sufficiently expandand lock tightly within the housing without the need of any thermallyconductive bonding agent. The second temperature should not exceed theupper temperature limit of the probe which is dictated by the uppertemperature limit of the thin film RTD element. If the secondtemperature exceeds the upper temperature limit of the thin film RTD,the thin film RTD element may have to be recalibrated, or may bepermanently damaged. A damaged element cannot be recalibrated since itmay not respond linearly due to delamination between the thin film RTDelement and the associated substrate.

FIG. 1 illustrates a length-wise, cross-sectional view of a temperaturesensor probe 10 in accordance with an aspect of the present invention.The probe 10 includes an outer housing 12 (or sheath) defining a bore 14therein that acts as a protective shield, and is made of a firstthermally conductive material, such as inconel, stainless steel,hastelloy, copper, brass, or any variations of an alloy of inconel,stainless steel, hastelloy, copper, brass or other conductive alloy. Thehousing 12 is cylindrical in this example because that shape lendsitself to a more desirable configuration for certain applications. Ofcourse, in other examples, the shape of the housing 12 can be differentto be more conducive for that particular application. Furthermore, thediameter, wall thickness and length of the housing 12 can be applicationspecific. In an aspect of the invention, the housing 12 has an outerdiameter of ⅛ of an inch, an internal diameter of 0.095 inches and alength of 6 inches.

The housing 12 can have a closed end 16 and an open end 18. The closedend 16 can be closed via an end cap welded to and closing off one end ofthe housing 12. In one aspect of the invention, the end cap has athickness of 0.01 inches. Alternatively, the housing 12 can be a singleintegrated assembly with the internal diameter being machined (e.g., viaan Electrical Discharge Machining (EDM) apparatus) to form the innerbore 14 from a solid cylindrical piece of material. An RTD element or athin film sensor (e.g., thin film precision RTD element) (not shown) ismounted within an insert 20 that is inserted intimately close to theclosed end 16 of the housing 12. The insert 20 can have a cylindricalshape with an inner cavity formed therein for housing the RTD element orthin film sensor.

A pair of connecting leads 22 is electrically coupled to the RTD elementor thin film sensor in the insert 20, and extend therefrom. Theconnecting leads can be non-insulated bare wire made of 95% Au and 5%Pd. A cylindrical insulating member 24 (e.g., a ceramic insulator) ispositioned within the housing 12 proximate the sensor and the insert 20and extends the length of the housing 12, as shown. The insulatingmember 24 can be secured inside the housing 12 by a high temperaturefiller 26, which can be applied at room temperature and then cured atelevated temperature limited by the upper operational temperature of theRTD element or thin film sensor. The insulating member 24 with theembedded RTD element or thin film sensor is bonded together via a hightemperature bonding agent 28 to protect and strengthen the probe 10 insevere vibration environments (e.g., up to about 20 g or higher). Theleads 22 extend from the RTD element through the insulating member 24and out of the open end 18 of the housing 12. The leads 22 can beelectrically coupled to an electrical receptacle (not shown) mounted tothe open end 18 to provide a constant precision current signal to thesensor within the insert 20. Although, a two wire sensor configurationis illustrated for the probe 10, it is to be appreciated that a 3-wireor 4-wire configuration can be employed for higher precisionmeasurements.

The insert 20 is formed of a second thermally conductive material thathas a thermal coefficient of expansion that is greater than the firstthermally conductive material. The insert 20 provides a good thermalcontact between the sensor in the insert 20 and the housing 12. Theinsert 20 can be formed of substantially pure silver, which provides fora good thermal conductor, as well as having a relatively high thermalcoefficient of expansion. Alternatively, the insert 20 can be formedfrom annealed copper, gold, aluminum, tungsten, molybdenum or magnesiumas long as the thermal coefficient of expansion of the insert materialis greater than the thermal coefficient of expansion of the housing 12.It is to be appreciated that the type of material selected for theinsert may affect the time constant proportionally to the applicableheat conduction coefficient.

The insert 20 has an outer diameter that is substantially the same orsubstantially equal as the internal diameter (e.g., within +/−0.0001″)of the housing 12 at a first temperature. The first temperature can be,for example, a temperature that falls within a range of about −40° F.(−40° C.) to about −100° F. (−73.33° C.) (e.g., −70° F. (−56.67° C.)).Therefore, the insert 20 can be inserted snugly into the bore 14 of thehousing 12 and disposed at the closed end 16 of the housing 12 when boththe housing 12 and the insert 20 are subjected to the first temperature.The housing 12 and the insert 20 can then be removed from the firsttemperature environment (e.g., a freezer), and exposed to a secondtemperature environment that is higher than the first temperatureenvironment.

As the ambient temperature rises, the insert 20 will expand at a fasterrate than the housing 12 resulting in an extremely tight fit of theinsert 20 in the housing 12 at elevated temperatures and provide for arepeatable fast temperature constant associated with the RTD element(e.g., pockets of air that detrimentally affect the time constant of theassembly will be effectively eliminated and there is no need for anybonding agent). The second temperature can be, for example, a roomtemperature that is greater than or equal to about 32 F (0° C.). Thesecond temperature can be selected to sufficiently allow the insert 20to expand and lock tightly within the housing 12, and not exceed theupper temperature limit of the probe 10.

FIG. 2 illustrates a front view of a cylindrical insert assembly 30 inaccordance with an aspect of the invention, while FIG. 3 illustrates aplan view of the cylindrical insert assembly 30 of FIG. 2. Thecylindrical insert assembly 30 includes a thin film sensor device 32axially disposed in an inner rectangular cavity 42 at a top wall 38 of acylindrical insert 34. The cylindrical insert 34 includes an outercylindrical wall 36 having an outer diameter machined to besubstantially a same diameter as an internal diameter of a temperatureprobe housing at the above mentioned first temperature. The innerrectangular cavity 42 is machined to be dimensioned to house the thinfilm sensor device 32 (e.g., precision thin film platinum element). Thethin film sensor device 32 is placed vertically, or axially, in theinner rectangular cavity 42, and then potted and cured using a suspendedsilver potting compound 44. The suspended silver potting compound 44 canbe similar to a compound used in U.S. Pat. No. 6,592,253, entitled,“Precision Temperature Probe Having a Fast Response”, the entirecontents of which are incorporated herein. It is to be appreciated thatcavity 42 can be circular, but a rectangular cavity using EDM machiningtechnology, would provide for a better fit in the shape of the thin filmsensor device 32 and reduce the amount of suspended silver pottingcompound needed, which can affect the time constant. A bottom wall 40defines a closed end of the cylindrical insert 34.

The closed end of the cylindrical insert 34 and the outer cylindricalwall 36 can have a surface that substantially contacts an inner boresurface of a housing of a temperature probe end of a temperature probeto facilitate temperature transfers from the temperature probe housingto the thin film sensor device 32. The thin film sensor device 32 caninclude a ceramic substrate on which a resistive element is mounted andcovered by an insulator. In one embodiment, by way of non-limitingexample, the thin film sensor device can have a thickness of 1.3 mm, awidth of 2.0 mm and a length of 2.3 mm. The resistive element can be a100 ohm platinum resistive element, but can be formed of othermaterials, such as platinum, nickel, nickel-iron, copper and others.

In view of the foregoing structural and functional features describedabove, a method will be better appreciated with reference to FIG. 4. Itis to be understood and appreciated that the illustrated actions, inother embodiments, may occur in different orders and/or concurrentlywith other actions.

FIG. 4 illustrates a methodology 50 for fabricating a temperature probeassembly in accordance with an aspect of the present invention. At 52, ahousing or sheath of a first thermally conductive material is machinedto define a bore that extends through a portion of the housing with apredetermined internal diameter, for example within ±0.0001″, at a firsttemperature. The first temperature can be, for example, a temperaturethat falls within a range of about −40° F. (−40° C.) to about −100° F.(−73.33° C.) (e.g., −70° F. (−56.67° C.)). The first thermallyconductive material can be, for example, inconel, stainless steel,hastelloy, copper, brass, or any variations of an alloy of inconel,stainless steel, hastelloy, copper, brass or other conductive alloy. Thehousing can be cleaned; the internal walls of the housingelectropolished; and the housing can again be cleaned to remove debriscaused by the electropolishing. The outer surface of the insert can alsobe electropolished to ascertain an intimate and tight contact with theinner surface of the housing.

At 54, an insert of a second thermally conductive material is machinedor stamped with a predetermined outer diameter at the first temperature,for example within +/−0.0001″, that substantially matches the internaldiameter of the housing at the first temperature. Additionally, an innerrectangular cavity is formed in the insert that is dimensioned to housea thin film precision RTD element. The second thermally conductivematerial has a thermal coefficient of expansion that is greater than thefirst thermally conductive material. The insert can be formed of silver,annealed copper, gold, aluminum, tungsten, molybdenum or magnesium aslong as the thermal coefficient of expansion of the insert material isgreater than the thermal coefficient of expansion of the housing. Theinsert can be cleaned; the outer walls of the insert electropolished;and the insert can again be cleaned to remove debris caused by theelectropolishing. The outer surface of the insert can also beelectropolished to ascertain an intimate and tight contact with theinner surface of the housing. The methodology 50 then proceeds to 56.

At 56, an RTD element is inserted into the cavity of the insert, bondedand cured to form an insert assembly, for example, employing a suspendedsilver potting compound, such as a type disclosed in U.S. Pat. No.6,592,253, the entire contents of which is incorporated herein. The timeconstant of the RTD element can then be verified. At 58, the signalleads are installed through a ceramic insulator and coupled to the RTDelement. At 60, the housing and the insert assembly are stabilized atthe first temperature. At 62, the insert assembly with the thin film RTDelement can be inserted into the bore of the housing and disposed at thebottom or closed end of the housing to form a temperature probeassembly. At 64, the temperature probe assembly is removed from thefirst temperature, and exposed to a second temperature that is higherthan the first temperature. The second temperature can be, for example,a room temperature that is greater than or equal to about 32° F. (0°C.). The second temperature can be selected to sufficiently allow theinsert to expand and lock tightly within the housing. It is to beappreciated that the upper temperature limit of the probe is dictated bythe upper temperature limit of the thin film RTD element, such the thinfilm RTD element would have to be recalibrated if it exceeds the uppertemperature limit of the probe. Other materials used for the RTDelements could include nickel, nickel-iron, copper and others.

The time constant of the RTD element can then be verified. The differentcoefficients of thermal expansion of the insert and the housing willpositively and very tightly lock the thin film precision RTD elementinsert at the bottom of the housing without any trapped air pockets. Thelocking will tighten with increasing temperature, making the formationof minute cracks with air due to thermal shock, thermal cycling,vibration and shock very unlikely and for all practical purposesimpossible. Sealing compound can then be provided to support the ceramicinsulator. The time constant of the RTD element can then be verified.The methodology then proceeds to 66 to perform additional temperatureprobe tests, such as thermal shock and thermal cycling tests andvibration and shock tests to verify the time constant of the RTD elementhas not degraded as a result of temperature or vibration stresses.

What have been described above are examples of the present invention. Itis, of course, not possible to describe every conceivable combination ofcomponents or methodologies for purposes of describing the presentinvention, but one of ordinary skill in the art will recognize that manyfurther combinations and permutations of the present invention arepossible. Accordingly, the present invention is intended to embrace allsuch alterations, modifications and variations that fall within thespirit and scope of the appended claims.

1-10. (canceled)
 11. A method for fabricating a temperature probeassembly, the method comprising: forming an inner bore in a housing of afirst thermally conductive material, the inner bore having an internaldiameter at a first temperature; forming an insert of a second thermallyconductive material having an outer diameter that is substantially equalto the internal diameter at the first temperature and an inner cavitydimensioned to house a temperature sensor; bonding a temperature sensorin the inner cavity; inserting, the insert into the inner bore of thehousing at the first temperature to form a temperature probe assembly;and exposing the temperature probe assembly to a second temperature thatis greater than the first temperature, wherein the second thermallyconductive material has a thermal coefficient of expansion that isgreater than the first thermally conductive material, such that theinsert is tightly locked in the inner bore at the second temperature.12. The method of claim 11, wherein the insert is formed of one ofsilver, annealed copper, gold, aluminum, tungsten, molybdenum andmagnesium.
 13. The method of claim 12, wherein the housing is formed ofone of any variation in alloys of inconel, stainless steel, hastelloy,copper, brass or other conductive alloy.
 14. The method of claim 11,wherein the first temperature is in a range of about −40° F. (−40° C.)to about −100° F. (−73.33° C.) and the second temperature is greaterthan or equal to about 32° F. (0° C.).
 15. The method of claim 11,wherein the temperature sensor is a thin film precision resistivetemperature device (RTD) element comprised of one of platinum, nickel,nickel-iron, copper or other suitable metals.
 16. The method of claim11, wherein forming an inner bore in a housing of a first thermallyconductive material comprises machining the inner bore with ElectricalDischarge Machining (EDM) apparatus and the forming of an insert of asecond thermally conductive material having an outer diameter that issubstantially equal to the internal diameter at the first temperatureand an inner cavity dimensioned to house a temperature sensor comprisingmachining the outer diameter with an EDM apparatus and machining theinner cavity with an EDM apparatus having a rectangular shape.
 17. Amethod for fabricating a temperature probe assembly, the methodcomprising: machining an inner bore in a housing of a first thermallyconductive material, the inner bore having an internal diameter at afirst temperature having a range of about −40° F. (−40° C.) to about−100° F. (−73.33° C.); machining or stamping an insert of a secondthermally conductive material having an outer diameter that issubstantially equal to the internal diameter at the first temperatureand an inner cavity dimensioned to house a resistive temperature device(RTD) element; bonding the RTD element in the inner cavity with asuspended silver potting compound; coupling signal wires to the RTDelement; inserting the insert into the inner bore of the housing at thefirst temperature to form a temperature probe assembly; and exposing thetemperature probe assembly to a second temperature greater than or equalto about 32° F. (0° C.), wherein the second thermally conductivematerial has a thermal coefficient of expansion that is greater than thefirst thermally conductive material, such that the insert is tightlylocked in the inner bore at the second temperature.
 18. The method ofclaim 17, wherein the insert is formed of one of silver, annealedcopper, gold, aluminum, tungsten, molybdenum and magnesium.
 19. Themethod of claim 17, wherein the housing is formed of one of anyvariation in alloys of inconel, stainless steel, hastelloy, copper,brass or other conductive alloy.
 20. The method of claim 17, wherein themachining the inner bore comprises machining the inner bore with anElectrical Discharge Machining (EDM) apparatus and the machining orstamping the insert comprises machining the outer diameter with an EDMapparatus and the inner cavity having a rectangular shape with an EDMapparatus.